Under appropriate conditions, many bacterial species produce an extracellular polysaccharide-containing layer which forms a "capsule" up to 10 .mu.m in thickness that surrounds the cells in a tight matrix. Some organisms produce a loose, amorphous "slime layer" that is more easily deformed than the relatively tight matrix of capsules. Slime layers also tend to be more easily deformed than capsules, and unlike capsules will not exclude particles. While the terms capsule and slime layer are often used in reference to these extracellular polysaccharide containing layers, some researchers refer to both structures as a "glycocalyx" (ie., glycocalyx is a more general term).
In addition to various polysaccharides and polysaccharide derivatives, the capsule may also contain glycoproteins. This capsule can be stained (e.g.,with Alcian blue) or be observed as a clear zone around the cells in an India ink wet mount. If the polysaccharide is soluble, it may diffuse throughout the culture media in which organisms are growing, forming slime, and making liquid media very viscous. The capsule may also help prevent desiccation of cells.
In vivo, capsular material is anti-phagocytic, and plays a role in pathogenicity of some organisms, including species such as Streptococcus pyogenes, S. pneumoniae, and Bacillus anthracis. In addition, glycocalyx material often plays important roles in pathogenesis, as it is involved in the attachment of microorganisms to host cells. In addition, capsular material allows microorganisms to attach to other surfaces, such as catheters and implants. Also, capsular material helps block the action of antimicrobials and causes biofouling in industrial processes. Thus, the production of extracellular capsular material provides microorganisms with virulence mechanisms, as well as allowing them to survive better in certain environments.
In addition to the pathogenesis implications of capsule production, capsular material often interferes with the metabolic reactions used to identify bacterial strains. With some organisms, such as Bacillus species, the problem is especially severe and the capsule can make it difficult to even obtain uniform suspensions of organisms for testing. Despite advances in technology, there remains a general need for systems that provide rapid and reliable biochemical identifications of microorganisms. For example, difficulties in identifying organisms of importance such as mucoid strains of Pseudomonas cepacia (now Burkhrolderia cepacia) in cystic fibrosis patients have occurred (See e.g,. Roman et al, ASM Abstracts, Abstract C-222, American Society for Microbiology, Washington, D.C., [1991], p. 379). In particular, it has been very difficult to develop an identification system which is capable of identifying various diverse types of organisms, while avoiding problems associated with the presence of capsular material produced by various organisms.